1. Field of the Invention
The present invention relates to a controller for a welding machine in which workpieces are pinched between a pair of electrodes through which welding current is supplied to the workpieces to cause the workpieces to be heated and welded.
2. Description of the Prior Art
In such a type of a welding machine, a cut-off timing of welding current must be properly controlled. An earlier setting of the timing may cause insufficient welding, whereas a later setting of the timing may cause welding time to be unnecessarily prolonged, resulting in that productivity is lowered, the electrodes are excessively worn, and welding strength and welding appearance are degraded due to occurrence of spattering. To comply with the problem, Japanese Patent Publication No. 62-18273 discloses a technique in which welding current is contrived to be cut off when .DELTA.R reaches a predetermined value. Here, .DELTA.R equals R(MAX)-R(PRESENT) where R(MAX) shows the maximum value of the interelectrode resistance during welding and R(PRESENT) shows a present value of the interelectrode resistance. The interelectrode resistance has a tendency to increase one time and decrease thereafter, since it increases with rise in the workpiece temperature and decreases with growth of nugget. Therefore, in Japanese Patent Publication No. 62-18273, welding current is cut off when the present resistance value R(PRESENT) decreases by the predetermined amount from the maximum resistance value R(MAX), judging that nugget has grown up.
According to the prior arts, a decreased amount of resistance, namely a predetermined value regarding .DELTA.R, is required. In this case, of course, the predetermined value on the decreased amount of the resistance must be changed in correspondence with variations in, for example, material, thickness, number, coating layer or others of the workpieces.
Therefore, a proper reference value on the decreased amount of the resistance must be experimentally obtained or calculated for each welding condition of the welding machine, causing a substantial burden to a user, manufacture or dealer of such a welding machine. Particularly in these days, as workpieces have a tendency of variously changing in material, thickness, coating layer or others, determination of a reference value for each welding condition has caused a considerable burden.
In order to meet various conditions, reference values corresponding to respective welding conditions must be stored, causing requirement of a large memory as well as complicated operation for controlling it which may disadvantageously degrade facility of the machine.
Even if the workpieces are uniform, repeated welding operation may cause changes with time such as increase of diameter of the electrodes, and process for correcting such a change must be preliminarily prepared for each welding condition.